Method for measuring stress in a structural element

ABSTRACT

Displacement measuring means ( 15, 16 ) are fixed onto the structural element. A hole ( 20, 21 ) is pierced in the measurement zone and a supply pressure is applied to a flat actuator introduced into the hole. The displacements measured are analyzed as a function of the supply pressure to determine the stress from a supply pressure which roughly compensates for the deformation of the element due to the piercing of the hole. The measuring means comprise two arms ( 15 ) that are fixed to the element at two respective anchoring points (X) aligned parallel to a measuring direction, and at least two displacement sensors ( 16 ) mounted on the arms on each side of the anchoring points to measure the variations in separation between the anchoring points. The arms ( 15 ) leave between them a gap through which the hole is pierced at a central position with respect to the anchoring points.

[0001] The present invention relates to a method for measuring stress ina structural element.

[0002] It applies in particular, although not exclusively, to themeasurement of residual prestress in a component of a concrete structuresubjected to a bending force due to the load on the structure and, onthe other hand, to a compressive prestress. Such a component istypically a prestressed concrete beam.

[0003] A prestressed beam comprises steel cables tensioned between theirends, with enough force that the concrete of the beam is subjected onlyto compressive stresses, in spite of the bending forces which are due tothe loads it supports.

[0004] Over the course of time, the tension in the prestressing cablestends to decrease, which means that the compressive stresses generatedby these cables may become insufficient to compensate for the tensilestresses due to the bending of the beams. These tensile stresses maylead to cracking of the concrete or even to the breaking of the beams.

[0005] It is therefore useful to be able to monitor the residual valueof the compressive stresses generated by the prestressing cables, so asto be able to take appropriate action should that prove necessary.

[0006] French Patent 2 717 576 describes a method of measuring aresidual prestress in a reinforced concrete beam which is subjected to avertical bending force and to a compressive longitudinal prestressforce, any section of the beam having a transverse line, known as theneutral axis of bending, along which the bending forces generate neithertensile stress nor compressive stress. This known method comprises thefollowing steps:

[0007] determining the position of the neutral bending axis in a givensection of the beam;

[0008] boring along said neutral bending axis, passing transverselythrough the beam, this boring giving rise to a certain elasticdeformation of the beam in its vicinity;

[0009] measuring the deformation of the beam near the boring, withrespect to an initial state prior to the bore hole being pierced;

[0010] introducing into the bore hole a hydraulic actuator comprisingtwo roughly semicylindrical shells which occupy roughly the entire crosssection of the bore hole and which are designed to move apart when theactuator is pressurized, this actuator being arranged in such a way thatthe two shells can move apart parallel to the prestress force;

[0011] pressurizing the hydraulic actuator while at the same timemeasuring the deformation of the beam near the bore hole;

[0012] recording the hydraulic pressure of the actuator whichcorresponds to the deformation of the beam due to the bore hole beingcanceled;

[0013] and determining the mean residual prestress along the neutralbending axis from the hydraulic pressure value thus measured.

[0014] One object of the present invention is to improve this method, byallowing better control over the relationships between the stress andthe displacements measured.

[0015] The invention thus proposes a method for measuring stress in astructural element, comprising the following steps:

[0016] fixing displacement measuring means onto the element in ameasurement zone;

[0017] piercing a hole in the element in the measurement zone;

[0018] introducing an actuator into the hole;

[0019] applying a supply pressure to the actuator; and

[0020] analyzing the displacements measured as a function of theactuator supply pressure so as to estimate the degree of stress in theelement in the measurement zone.

[0021] According to the invention, the displacement measuring meanscomprise two arms that are fixed to the element at two respectiveanchoring points aligned parallel to a measuring direction, and at leasttwo displacement sensors mounted on the arms on each side of theanchoring points and each measuring a relative displacement, parallel tothe measuring direction, of two respective portions of the arms whichportions lie facing one another. The arms leave between them a gapthrough which the hole is pierced at a central position with respect tothe anchoring points.

[0022] Thus, the anchoring points which act as a basis for thedisplacement measurements are positioned optimally with respect to thehole and to the measuring direction, without this in any way impedingthe boring of the hole and the instrumenting of the measurement zone.

[0023] The method makes it possible in a particularly advantageous wayto constantly record the displacement and supply pressure measurementswhile the hole is being pierced and the supply pressure is being appliedto the actuator, this allowing in-depth analysis of the results.

[0024] In a preferred embodiment, the hole comprises a slot orientatedat right angles to the measuring direction, symmetrically with respectto an axis passing through the anchoring points, the actuator being aflat actuator introduced into the slot.

[0025] The fact that the slot and the measurement axis are at rightangles to each other and the fact that the slot is centered with respectto the anchoring points improve the reliability of the displacementmeasurements and improve their correlation with the looked-for stress.

[0026] This slot may pass all the way through the element, but that isnot essential as long as it is deep enough.

[0027] The flat actuator may be supplied with hydraulic fluid by amanually operated pump and associated with means of measuring the supplypressure.

[0028] The flat actuator may be introduced into the slot with theinsertion of at least one wedging plate which makes the distribution ofthe force exerted by the actuator uniform over the extent of the slot.

[0029] The displacements analyzed advantageously represent a variationin separation between the two anchoring points, which variation isobtained from a mean of the displacements respectively measured by thesensors. In a preferred embodiment, additional displacement measuringmeans are fixed to the structural element at two anchoring points lyingoutside of the measuring zone and aligned in the measuring direction andhaving, between them, a distance roughly identical to the distancebetween the two anchoring points lying in the measurement zone. Theseadditional measurement means provide a corrective term that represents avariation in separation between the two anchoring points lying outsidethe measurement zone, said corrective term being subtracted from saidmean of the displacements in the analysis step.

[0030] In some particular embodiments of the method:

[0031] the displacement sensors have a measurement accuracy of the orderof one micron;

[0032] the supply pressure of the actuator is increased until a supplypressure is achieved that roughly compensates for the deformation of theelement that is due to the piercing of the hole, then the supplypressure has gradually reduced while continuing to record thedisplacement measurements, and a degree of compression in themeasurement zone is estimated from the supply pressure which has roughlycompensated for the deformation of the element;

[0033] the change in the measured displacements is recorded as afunction of the supply pressure of the actuator, and if a supplypressure which roughly compensates for the deformation of the elementdue to the piercing of the hole is not achieved, the change in themeasured displacements is extrapolated so as to estimate the degree ofstress in the element in the measurement zone; if extrapolation istoward high pressures, it can then be determined that the measurementzone is in a state of compression, while it can be determined that themeasurement zone is in a state of tension, if extrapolation is towardnegative pressures;

[0034] once the measurements have been taken, an actuator containing asubstance under pressure is left in the hole;

[0035] use is made of a measurement zone situated roughly along theneutral axis of the structural element;

[0036] use is made of at least two measurement zones situated roughlysymmetrical with respect to the neutral axis of the structural element,and in which a stress is evaluated along the neutral axis using a meanof the stresses measured in said measurement zones.

[0037] Other specifics and advantages of the present invention willbecome apparent from the description hereinafter of some nonlimitingembodiments, with reference to the appended drawings, in which:

[0038]FIG. 1 is a front view of a beam on which stress measurements willbe taken according to the invention;

[0039]FIG. 2 is a schematic front view of displacement measuring meanslying in the stress measurement zone;

[0040]FIG. 3 is a schematic front view of other displacement measuringmeans lying outside of the stress measurement zone;

[0041]FIG. 4 is a sectional view of the measurement zone;

[0042]FIG. 5 is a block diagram of a control and measurement sequencethat can be used in the method; and

[0043]FIG. 6 shows an example of a graph generated according to themethod.

[0044]FIG. 1 illustrates the application of the invention to themeasurement of the residual prestress in a prestressed reinforcedconcrete beam 10. On its top face, this beam 10 is subjected to variableloads which tend to cause it to bend, in additional to its self-weight.In consequence, in the upper part of the beam, the loads inducecompressive (positive) stresses in the concrete, whereas they inducetensile (negative) stresses in the concrete of the lower part of thebeam.

[0045] The line B in FIG. 1 indicates the neutral axis of the beam, thatis to say the axis for which the stresses induced by the loads itsupports and by its self weight change sign. For the concrete to workproperly, prestressing cables longitudinally compress the beam so thatthe overall stress along the neutral axis B corresponds to a positivecompressive stress.

[0046] In FIG. 1, the line A indicates an axis of symmetry of the beam10 and the point C denotes its intersection with the neutral axis B.

[0047] To measure the residual stress in the concrete at the neutralaxis (point C) use is made, in the example depicted in FIG. 1, of twomeasurement zones Z arranged symmetrically with respect to the point C.

[0048] In each measurement zone Z, a value of the compressive stressparallel to the direction B is determined. To obtain the value of theresidual stress at the point C, all that is required is for anarithmetic mean of the two measured values to be calculated.

[0049] If the point C is accessible, it is also possible to take justone measurement in its immediate vicinity.

[0050] A first step in the stress measuring method consists in definingtwo anchoring points X in the measurement zone Z. These two anchoringpoints are aligned parallel to the measuring direction B and theirdistance is defined precisely using a template 11 with two openingsthrough which holes are pierced, in which holes anchor bolts areinstalled.

[0051] Each measurement zone Z is associated with a reference zone Z′ inwhich two other anchoring points X′ are defined, these too being alignedparallel to the direction B and having between them the same distance asthere is between the anchoring points X. The anchoring points X′ may bepositioned using a template 12 similar to the one used in themeasurement zone Z.

[0052] The next step consists in equipping the measurement zone Z withdisplacement measuring means such as those depicted in FIG. 2.

[0053] These means comprise two arms 15 fixed respectively in theircentral part to rods sealed into the holes pierced in the concrete atthe anchoring points X. The arms 15 run in a direction which isgenerally at right angles to the measuring direction B. Their ends arebent inward, and displacement sensors 16 are inserted between the facingportions 17 of the bend ends of the arms 15.

[0054] The sensors 16 may be electromechanical feelers with ameasurement accuracy of the order of one micron in a range ofdisplacement of ±1 mm.

[0055] The two sensors 16 are arranged symmetrically with respect to theaxis B′ passing through the two anchoring points X, which is parallel tothe measuring direction B. Thus, the arithmetic mean (d₁+d₂)/2 of thetwo displacement measurements d₁, d₂ supplied by the sensors 16represents a measure of the variation in separation between the twoanchoring points X.

[0056] Additional displacement measuring means (FIG. 3) are installed inthe reference zone Z′. These means comprise two platelets 18 fixedrespectively to rods sealed into the holes pierced in the concrete atthe two anchoring points X′. A displacement sensor 19 similar to those16 provided in the measurement zones Z is arranged between the twoplatelets 18. The displacement value d₃ supplied by this sensor 19represents the variation in separation between the two anchoring pointsX′.

[0057] The next step in the method consists in piercing a hole 20, 21 inthe concrete element 10 in the measurement zone Z. This hole is piercedthrough the gap left free between the two arms 15 of the displacementmeasuring device, as illustrated in dotted line in FIG. 2.

[0058] This hole comprises a slot 20 directed at right angles to themeasuring direction B. This slot 20 is centered between the twoanchoring points X, and the axis B′ passing through these two anchoringpoints X passes through the middle of the slot 20. The slot 20 may havea thickness of the order of one centimeter and a length of about tentimes that.

[0059] At the two ends of the slot 20, the hole made in the measurementzone Z has two cylindrical bore holes 21, the diameter of which is, forexample, of the order of a few centimeters.

[0060] To drill the hole 20, 21, the bore holes 21 are made first of allusing a hydraulic concrete core drill, then the slot 20 is made using ahydraulic concrete cutter. The core drill and the cutter may be mountedon a chassis that is anchored to the concrete structure.

[0061] The hole 20, 21 may pass right through the beam 10. In somecases, it may penetrate the concrete to a sufficient depth withoutpassing through it.

[0062] The next step (FIG. 4) consists in introducing a flat actuator 24into the slot 20. This actuator may consist of two metal plates weldedtogether along their periphery. An injection orifice, not depicted,causes the space between the two plates to communicate with a hydrauliccircuit. As shown by FIG. 4, a wedging sheet 25 of appropriate thickness(or a stack of several sheets) may be placed, with the flat actuator 24,in the slot 20. This plate 25 allows more uniform distribution of thethrust exerted by the flat actuator 24 across the extent of the slot 20.

[0063] The hydraulic circuit is depicted schematically in FIG. 5. Thehydraulic fluid from the reservoir 27 is sent under pressure to the flatactuator 24 by a pump 28. By way of example, the supply pressures mayrange up to about 200 to 300 bar. A pressure gauge 29 situated betweenthe pump 28 and the actuator 24 is used to measure the actuator supplypressure. To reach high pressures with a gradual pressure rise, the pump28 is advantageously manually operated.

[0064]FIG. 5 also shows a computation device 30 consisting, for example,of a portable computer of the PC type, which gathers the variousparameters measured by the sensors 16, 19 and 29. The computation device30 exploits the displacement and pressure measurements to evaluate thestress exerted in the measurement zone Z. This exploitation may be donein real time, which means that the stress measurement is availableimmediately.

[0065] The data recorded by the computation device 30 correspond to thechange in the separation between the two anchoring points X as afunction of the supply pressure P applied to the flat actuator 24. Theseparation (d₁+d₂)/2 measured between the two anchoring points X iscorrected using the separation d₃ measured between the anchoring pointsX′. The relevant displacement variable is therefore (d₁+d₂)/2−d₃.

[0066] An example of the change in this variable (d₁+d₂)/2−d₃ as afunction of pressure is illustrated by the graph in FIG. 6. Curves I andII correspond respectively to the rise in pressure in the flat actuator24 and to the fall in pressure. The rise is halted when the measuredvariable (d₁+d₂)/2−d₃ reaches the displacement value d₀ that correspondsto the value recorded before the hole was pierced. The pressure Pmeasured at that instant corresponds to the looked-for compressivestress.

[0067] If the displacement value d₀ has not yet been reached when theactuator 24 is supplied with its maximum pressure, then the computer 30extrapolates the curve obtained, which is approximately linear, toobtain a measure of the stress given by the X-axis value of the point ofintersection of the extrapolated line with the y-axis value d₀.

[0068] The variations in displacement as a function of supply pressureare also recorded while the hole 20, 21 is being pierced, and this makesit possible to observe the behavior of the structure and possibly toestimate how deep the holes need to go, this being the depth beyondwhich the additional displacements measured at the anchoring points areno longer significant.

[0069] The fact of arranging the flat actuator 24 in a slot 20perpendicular to the measuring direction B allows reliable measuring inthat direction, avoiding the geometric configuration of the hole causingother undesirable stresses to be taken into consideration. The boreholes 21 at the ends of the slots 20 limit parasitic stresses at theends and make the slot 20 easier to cut. They may also make it easierfor the equipment to be fitted.

[0070] The making of the hole 20, 21 does not generally disturb thestructure, given its small size. However, if such disturbance is feared,this may be overcome by leaving in the hole, once the measures have beentaken, an actuator containing a pressurized substance. This substanceis, for example, a resin injected into the flat actuator 24 at apressure corresponding to the measured residual stress, which is left tocure in the actuator which will remain in situ.

[0071] It should be noted that the method may also be applied when themeasurement zone is in a state of tension rather than of compression. Inthis case, the relaxation that follows the piercing of the hole tends tocause the arms 15 to move apart rather than to move closer together asis the case with compressive stresses (FIG. 6). Furthermore, when theactuator is supplied, it introduces an additional separation which movesthe measurement point (d₁+d₂)/2−d₃ even further away from the referencevalue do. This does not prevent the degree of stress in the measurementzone from being estimated using the aforementioned process ofextrapolation. Quite simply, extrapolation is toward the negativepressures (rather than toward the higher pressures). The opposite X-axisvalue at which the extrapolated measurement straight line reaches they-axis value d₀ in a diagram according to FIG. 6 gives an estimate ofthe tensile stress.

[0072] Furthermore, the method can be applied to all types of structure,which are not necessarily made of concrete, for example to stoneworkstructures.

1. A method for measuring stress in a structural element (10),comprising the following steps: fixing displacement measuring means (15,16) onto the element in a measurement zone (Z); piercing a hole (20, 21)in the element in the measurement zone; introducing an actuator (24)into the hole; applying a supply pressure to the actuator; and analyzingthe displacements measured as a function of the actuator supply pressureso as to estimate the degree of stress in the element in the measurementzone, characterized in that the displacement measuring means comprisetwo arms (15) that are fixed to the element at two respective anchoringpoints (X) aligned parallel to a measuring direction (B), and at leasttwo displacement sensors (16) mounted on the arms on each side of theanchoring points and each measuring a relative displacement, parallel tothe measuring direction, of two respective portions (17) of the armswhich portions lie facing one another, and in that the arms (15) leavebetween them a gap through which the hole (20, 21) is pierced at acentral position with respect to the anchoring points.
 2. The method asclaimed in claim 1, in which the hole comprises a slot (20) orientatedat right angles to the measuring direction (B), symmetrically withrespect to an axis (B′) passing through the anchoring points (X) and inwhich the actuator is a flat actuator (24) introduced into the slot. 3.The method as claimed in claim 2, in which the hole furthermorecomprises two cylindrical bore holes (21) situated one at each of thetwo ends of the slot (20) and having a diameter greater than thethickness of the slot.
 4. The method as claimed in claim 2 or 3, inwhich the flat actuator (24) is supplied with hydraulic fluid by amanually operated pump (28) and is associated with means (29) ofmeasuring the supply pressure (P).
 5. The method as claimed in any oneof claims 2 to 4, in which the flat actuator (24) is introduced into theslot (20) with the insertion of at least one wedging plate (25).
 6. Themethod as claimed in any one of the preceding claims, in which thedisplacements analyzed represent a variation in separation between thetwo anchoring points (X), which variation is obtained from a mean of thedisplacements (d₁, d₂) respectively measured by the sensors (16).
 7. Themethod as claimed in claim 6, in which additional displacement measuringmeans (18, 19) are fixed to the structural element (10) at two anchoringpoints (X′) lying outside of the measuring zone (Z) and aligned in themeasuring direction (B) and having, between them, a distance roughlyidentical to the distance between the two anchoring points (X) lying inthe measurement zone, and in which the additional measurement meansprovide a corrective term (d₃) that represents a variation in separationbetween the two anchoring points (X′) lying outside the measurementzone, said corrective term being subtracted from said mean of thedisplacements in the analysis step.
 8. The method as claimed in any oneof the preceding claims, in which the displacement sensors (16, 19) havea measurement accuracy of the order of one micron.
 9. The method asclaimed in any one of the preceding claims, in which the displacementand supply pressure measurements are constantly recorded while the hole(20, 21) is being pierced and while the supply pressure is being appliedto the actuator (24).
 10. The method as claimed in any one of thepreceding claims, in which the supply pressure of the actuator (24) isincreased until a supply pressure is achieved that roughly compensatesfor the deformation of the element that is due to the piercing of thehole (20, 21), then the supply pressure is gradually reduced whilecontinuing to record the displacement measurements, and a degree ofcompression in the measurement zone is estimated from the supplypressure which has roughly compensated for the deformation of theelement.
 11. The method as claimed in any one of claims 1 to 9, in whichthe change in the measured displacements is recorded as a function ofthe supply pressure of the actuator, and if a supply pressure whichroughly compensates for the deformation of the element due to thepiercing of the hole is not achieved, the change in the measureddisplacements is extrapolated so as to estimate the degree of stress inthe element in the measurement zone.
 12. The method as claimed in claim11, in which the measurement zone is determined to be in a state ofcompression when extrapolation is toward high pressures, while themeasurement zone is determined to be in a state of tension whenextrapolation is toward negative pressures.
 13. The method as claimed inany one of the preceding claims, in which, once the measurements havebeen taken, an actuator containing a substance under pressure is left inthe hole.
 14. The method as claimed in any of the preceding claims, inwhich use is made of a measurement zone (Z) situated roughly along theneutral axis (B) of the structural element (10).
 15. The method asclaimed in any one of claims 1 to 13, in which use is made of at leasttwo measurement zones (Z) situated roughly symmetrical with respect tothe neutral axis (B) of the structural element (10), and in which astress is evaluated at the center of gravity using a mean of thestresses measured in said measurement zones.